A tropical cyclone is a low-pressure system which develops in the tropics and is sufficiently intense to produce sustained gale force winds of at least 63km/h. If the sustained wind reaches hurricane force of at least 118km/h the system is defined as a severe tropical cyclone.

A nuclear explosion occurs as a result of the rapid release of energy from an intentionally high-speed nuclear reaction. The driving reaction may be nuclear fission, nuclear fusion or a combination of the two.

A volcano is a vent or chimney which transfers molten rock known as magma from depth to the Earth's surface. Magma erupting from a volcano is called lava and is the material which builds up the cone surrounding the vent.

Emergency Management is represented by a range of measures which have been designed for the management of risks to communities and to the environment. It involves establishing plans, structures and arrangements to bring together the normal activities of governments with voluntary and private agencies.

The impact of a hazardous event depends on the elements at risk, such as; population or buildings and their associated vulnerability to damage or change as a result of the event. Estimating risk is an uncertain science because it involves forecasting events for which the time and the location may be largely unknown.